A control method for continuous current mode of flyback converter

Abstract

The present invention discloses a control method for a continuous current mode of a flyback converter. When the flyback converter operates in a critical current mode, the transformer current peak value is recorded as i pk1 And the corresponding off time T off1 , recorded as the first cycle; if the current peak value i of a certain cycle pk No higher than i pk1 , after the original shutdown logic is completed, the flyback switch is turned on, which is recorded as the second cycle; if the current peak value i pk Higher than i pk1 , when the difference is Δi, the switch is controlled to reduce the off time of the cycle to T off1 ‑k•Δi, the current cannot return to zero at the end of the cycle and enters the continuous current mode, which is recorded as the third cycle; after that, the current peak value i pk To determine the condition, the flyback converter switches between a discontinuous current mode (DCM) and a continuous current mode (CCM). The present invention can stably control and adjust the duty cycle of the flyback converter entering CCM, and take into account the output average current calculation.

Description

Technical Field

[0001] The present invention relates to the technical field of switch power supply chip control, and in particular to a control method for a continuous current mode of a flyback converter. Background Art

[0002] Switching power supply is the energy source of most electrical and electronic products, among which AC / DC conversion is the first step to obtain energy from the power grid. Generally speaking, AC / DC is a rectifier bridge plus DC / DC conversion. Figure 5 As shown, the alternating positive and negative sinusoidal AC power must first pass through a rectifier bridge to be converted into a positive full-wave voltage, and then pass through a filter capacitor to form a more stable voltage for the subsequent DC / DC converter. However, the grid voltage as an input voltage has a very wide range with regional changes: 80-280Vac, and the grid frequency will also fluctuate greatly with the load: 47-63Hz. This limits the use of filter capacitors. The withstand voltage must be able to withstand the highest input voltage, and the capacitance must be large enough to ensure the normal operation and performance of the subsequent DC / DC. Using a filter capacitor with too small a capacitance will increase the voltage fluctuation on it and lower the minimum voltage.

[0003] As the main topology of small and medium power AC / DC switching power supplies, flyback converters are widely used in various consumer electronic products. With the popularity of smartphones, chargers have put forward more stringent requirements on the power density of the whole power supply and the volume of each component. The filter capacitor after the rectifier bridge usually needs to use a 400V withstand voltage electrolytic capacitor, and the capacitance needs to reach more than 2uF / W of full load power. Its volume will account for 30-40% of the entire charger, making it the most difficult component to reduce the volume.

[0004] Figure 6 The following is a typical AC / DC flyback converter application circuit, where the input is AC voltage, BD is a rectifier bridge, C1 is a filter capacitor, Vbus is the voltage on C1, Q1 is the primary main switch, Q2 is the secondary rectifier switch, and the transformer T has a primary winding with N turns. p , the number of turns of a secondary winding is N s C2 is the output filter capacitor, RL is the output load, V o is the DC output voltage. When the flyback operates in CCM, Q1 operates with a pulse cycle with a duty cycle of D, and Vo / Vbus will satisfy N s / N p •D / (1-D). When a smaller C1 value is used to cause a significant change in the Vbus voltage, the flyback converter needs to be able to respond in time to a larger duty cycle and a higher peak current to ensure that the output voltage does not drop.

[0005] The existing peak current control technology can well control the flyback converter in a closed loop. Its basic block diagram is as follows: Figure 7 As shown, the control circuit samples the current flowing through the switch tube Q1, and compares it with the current peak value calculated by the loop through comparator 1. When the current exceeds this value, the comparator flips to turn off Q1. After that, the oscillator generates a changing periodic pulse signal, or combines with the auxiliary winding N a The resonance valley signal is detected to turn on Q1. This technology is simple to implement, not only to ensure that the current flowing through the transformer and Q1 is within a safe range, but also to achieve decoupling of the inductor current in the loop control, thereby improving the loop stability; but the disadvantage is that when the duty cycle D of CCM is greater than 50%, subharmonic oscillation will occur, and a fixed slope compensation needs to be added to the loop. On the one hand, this reduces the original peak current, and the compensation amount is sometimes insufficient or over-adjusted. On the other hand, the duty cycle will be limited to less than 80% due to the fixed oscillator frequency, making it impossible to keep the output voltage stable at a lower Vbus voltage.

[0006] Secondly, the existing peak current control technology also takes into account the calculation of the output average current to ensure the consistency of the overcurrent point under a wide range of input and output conditions, that is, primary constant current, without the need for optocoupler feedback, such as Figure 8 As shown in Figure 1, the area of ​​the transformer current drop phase needs to be averaged periodically to obtain the output average current. Figure 7 The auxiliary winding detects the demagnetization time of the transformer current, and then uses the triangle area formula; the calculation under CCM requires analysis of the complete transformer current signal, such as the starting value or midpoint value of the detection current trapezoid. The disadvantage of this calculation method is that the complete transformer current signal is easily affected by turn-on noise and sampling delay. If there is no relevant compensation measure, the calculation error of the output average current under CCM is more than 10%.

[0007] Based on the above considerations, a control method is needed to further reduce the C1 filter capacitance while taking into account the output average current calculation of DCM and CCM. Summary of the invention

[0008] The object of the present invention is to provide a control method for a continuous current mode of a flyback converter, which can stably control and adjust the degree and speed of the duty cycle when the flyback converter enters CCM, take into account the calculation of the output average current, and enable the flyback converter to use a smaller input filter capacitor in the application after the AC / DC rectifier bridge, reduce the output voltage ripple, and ultimately achieve a higher power density.

[0009] The object of the present invention can be achieved by the following technical solution to provide a control method for a flyback converter in continuous current mode, comprising the following steps:

[0010] Step 1: Based on the existing peak current control technology, when the flyback converter works in the critical current mode, the transformer current peak value is recorded as i pk1 and the corresponding off time T off1 , recorded as the first cycle;

[0011] Step 2: If the current peak value i of a certain cycle pk No higher than i pk1 , then the flyback switch is turned on after the original shutdown logic ends, which is recorded as the second cycle;

[0012] Step 3: If the current peak value i of a certain cycle pk Higher than i pk1 , when the difference is Δi, the switch is controlled to reduce the off time of the cycle to T off1 -k•Δi, the current cannot return to zero at the end of the cycle, thus entering CCM, which is recorded as the third cycle, where k is the preset proportional coefficient;

[0013] After that, the peak current i pk To determine the condition, the flyback converter switches between DCM and CCM.

[0014] As a further solution of the present invention: In step 1, the working state can be changed in a certain cycle, that is, by giving the current peak value of the cycle as i pk1 And turn on in critical current mode to obtain the off time T off1 .

[0015] As a further solution of the present invention: in step one, the critical current mode is not limited to turning on the switch immediately after the transformer current returns to zero, but also includes turning on the switch at the first valley bottom of the resonance, and the two time points differ by half a system resonance cycle.

[0016] As a further solution of the present invention: in step three, when the minimum input voltage of the flyback converter is lower, the required CCM duty cycle is larger, and the k value needs to be increased to reduce the off time of the third cycle more.

[0017] As a further solution of the present invention, the output average current calculation method is:

[0018]

[0019] Among them, i pk is the peak current of each cycle, Δi is i pk -i pk1 (positive or zero), N ps is the transformer primary to secondary turns ratio, V o is the flyback output voltage value, L m is the transformer excitation inductance, T offis the turn-off time or transformer current demagnetization time, T s For the cycle time.

[0020] As a further solution of the present invention: when in the first and second cycles, the current peak value i pk Less than i pk1 , Δi needs to be substituted as a zero value into the output average current calculation method, which is simplified to:

[0021]

[0022] As a further solution of the present invention: when calculating the output average current, when in the first and second cycles, T off is the transformer current demagnetization time; when in the third cycle, since the transformer current cannot return to zero at the end of the cycle, T off T off1 -k•Δi.

[0023] Beneficial effects of the present invention: The present invention stably controls and adjusts the degree and speed of the duty cycle when entering CCM by controlling the turn-off time of the flyback converter, taking into account the calculation of the output average current, thereby enabling the flyback converter to use a smaller-capacitance input filter capacitor in the application after the AC / DC rectifier bridge, reducing the output voltage ripple, and ultimately achieving a higher power density. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described below in conjunction with the accompanying drawings.

[0025] Figure 1 The transformer current waveform of the flyback converter in DCM and CCM according to the implementation method of the present application;

[0026] Figure 2 A flow chart for controlling the method for implementing the present application;

[0027] Figure 3 This is an example of control using the method of Example 1 of the present application;

[0028] Figure 4 This is an example of control using the method of Example 2 of the present application;

[0029] Figure 5 It is the filter voltage fluctuation curve under different filter capacitor values;

[0030] Figure 6 It is a typical AC / DC flyback converter circuit;

[0031] Figure 7 It is an example of peak current control of a flyback converter;

[0032] Figure 8Figure 2 is the transformer current waveform of the flyback converter in CCM and DCM. DETAILED DESCRIPTION

[0033] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0034] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work should fall within the scope of protection of the present invention.

[0035] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so as to describe the embodiments of the present invention described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0036] In this embodiment, a control block diagram of a continuous current mode of a flyback converter is provided, such as Figure 2 and 3 As shown, transformer current sampling uses R1 to convert the current signal into a voltage signal. Based on the existing peak current control technology, as shown in the dotted box, a sample holder 1, a comparator 2, subtractors 1 and 2, and proportional coefficients k and N are added. ps •V o / L m ; Output average current calculation unit adds ΔVi and proportional coefficient N ps •Vo / L m The output signal is omitted; the slope compensation unit is omitted, and the complete transformer current signal does not need to be introduced into the output average current calculation unit. When the demagnetization / valley detection unit detects that it is in critical current mode, the sample and hold 1 is enabled so that when the comparator 2 flips, T off Latched to T off1, realizing the first cycle; when in DCM, the sample and hold 1 does not work, the output ΔVi of the subtractor 1 is 0, realizing the second cycle; when it is necessary to enter CCM, ΔVi>0, the oscillator is based on the output T of the subtractor 2 off1 -k•ΔVi time to open Q1 and realize the third cycle. Based on this structure, it can be realized Figure 2 The output average current calculation unit does not need to distinguish between DCM and CCM.

[0037] The output average current calculation method can be based on Figure 1 The area 3 is derived in the figure. Since the falling slope of the transformer current is only affected by the output voltage and hardly changes, a parallelogram can be constructed to simplify the calculation of area 3. Part of the upper base of the trapezoid of area 3 is Δi, and the other part is proportional to the reduced off time k•Δi. This relationship is the falling slope of the transformer current N ps •V o / L m ; The bottom of the trapezoid is the current cycle i pk , high is the reduced T off , from which the output average current of this cycle can be calculated.

[0038] In a specific embodiment, the proportionality coefficient k is set to 0, which means that the third period is pk >i pk1 The off time will remain at T off1 The control structure of this embodiment can be further simplified, the subtractor 2 and the two proportional coefficients can be omitted, and the output average calculation unit is more streamlined.

[0039] In another embodiment, a control block diagram of a continuous current mode of a flyback converter is provided. Figure 4 As shown. The proportional coefficient k can be changed according to the working conditions; the proportional coefficient N ps •V o / L m is split into a real physical fixed quantity N ps / L m and the change V o Two coefficients. k and V o Both can be controlled by the demagnetization / valley detection unit. This unit can be controlled by the auxiliary winding N a Indirectly detect the input and output voltage information of the flyback converter, so as to reasonably change k and V o The coefficient can more accurately control the duty cycle of entering CCM to adapt to different applications.

[0040] The present invention does not limit the implementation of the addition, subtraction, multiplication and division in the output average current calculation method, nor does it limit any use of the calculation result, such as for loop control, power calculation, or triggering protection.

[0041] One of the core points of the present invention is that when using filter capacitors C1 of different capacitance values, the degree and speed of the duty cycle of the flyback converter when entering CCM can be stably controlled and adjusted by changing the proportional coefficient k when Vbus changes drastically. If a larger k value is used, the switching frequency will increase rapidly with the increase of the peak current, and will not be limited by the preset frequency of the oscillator, and the maximum duty cycle of CCM can exceed 85%.

[0042] One of the core points of the present invention is that since the off-time is controlled, no subharmonic oscillation with a large and a small off-time will be generated when entering CCM, no slope compensation unit is required, and there will be no loss of the maximum peak current. The utilization rate of the transformer is improved, and the adaptability of the flyback is enhanced.

[0043] One of the core points of the present invention is that when calculating the output average current, there is no need to distinguish between DCM and CCM operating conditions, nor is a complete transformer current signal required. Only peak current information is required, which avoids the influence of Q1 turn-on noise and sampling delay and can improve calculation accuracy.

[0044] It is to be understood that the above embodiments are merely exemplary embodiments used to illustrate the principles of the present invention, but the present invention is not limited thereto. For those of ordinary skill in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A control method for a flyback converter in continuous current mode, characterized in that: The following steps are involved: Step 1: Based on the peak current control technology, when the flyback converter works in the critical current mode, the transformer current peak value is recorded as i pk1 And the corresponding off time T off1 , recorded as the first cycle; Step 2: If the current peak value i of a certain cycle pk No higher than i pk1 , then the flyback switch is turned on after the original shutdown logic ends, which is recorded as the second cycle; Step 3: If the current peak value i of a certain cycle pk Higher than i pk1 , when the difference is Δi, the switch is controlled to reduce the off time of the cycle to T off1 -k·Δi, the current cannot return to zero at the end of the cycle, thus entering CCM, which is recorded as the third cycle, where k is the preset proportional coefficient; After that, the peak current i pk To determine the condition, the flyback converter switches between DCM and CCM.

2. The control method of a flyback converter continuous current mode according to claim 1, characterized in that: In step 1, the working state can be changed in a certain cycle, that is, by giving the current peak value of the cycle as i pk1 And turn on in critical current mode to obtain the off time T off1 .

3. The control method of a flyback converter continuous current mode according to claim 1, characterized in that: In step 1, the critical current mode is not limited to turning on the switch immediately after the transformer current returns to zero, but also includes turning on the switch at the first valley bottom of the resonance, and the difference between the two time points is half a system resonance cycle.

4. The control method of a flyback converter continuous current mode according to claim 1, characterized in that: In step 3, when the minimum input voltage of the flyback converter is lower, the required CCM duty cycle is larger, and the k value needs to be increased to reduce the off time of the third cycle more.

5. The control method of a flyback converter continuous current mode according to claim 1, characterized in that: The output average current is calculated as: Among them, i pk is the peak current of each cycle, Δi is i pk -i pk1 , N ps is the transformer primary to secondary turns ratio, V o is the flyback output voltage value, L m is the transformer excitation inductance, T off is the turn-off time or transformer current demagnetization time, T s For the cycle time.

6. The control method of a flyback converter continuous current mode according to claim 5, characterized in that: When in the first cycle and the second cycle, the current peak value i pk Less than i pk1 , Δi needs to be substituted as a zero value into the output average current calculation method, which is simplified to:

7. The control method of a flyback converter continuous current mode according to claim 4, characterized in that: When calculating the output average current, when in the first cycle and the second cycle, T off is the transformer current demagnetization time; when in the third cycle, since the transformer current cannot return to zero at the end of the cycle, T off T off1 -k·Δi.

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